This invention relates to thermionic cathodes and to a method for producing thermionic emitting materials for use therewith. In a more particular aspect, this invention concerns itself with a novel process for preparing a homogeneous mixture of finely divided alkaline earth carbonates especially adapted for use as an emission material for coating thermionic cathodes.
Thermionic vacuum tubes are well known electronic devices that rely on thermally emitted electrons from a heated electrode to produce tube current. The electrode is referred to as a cathode since it forms the negative electrode of the tube and is heated until it emits electrons. It can be heated directly by passing a current directly through the cathode or filament. Also, it can be heated indirectly by an independent heating element or ionic bombardment on the emitting surface.
Generally, the emitting surface is fabricated by applying an emitting material to the surface of a substrate as a coating rather than rely on the substrate material itself as the electron emitting portion of the tube. Since the emitting surface carries no heating current, there is no voltage drop along the surface. This fact, coupled with the high emission capabilities of the coated material, make the cathodes especially useful for modern day high power accelerations.
The successful utilization of thermionic tubes, however, depends to a great extent on the operational efficiency of the emitting cathode as well as the time consuming and expensive procedures which must be restored to if an efficiently operating device is to be fabricated. For example, mixtures of alkaline earth carbonates have been found to be useful and efficient as thermionic cathode coating materials. However, such mixtures lack homogenity and particle size uniformity. One technique for overcoming this problem is to subject the carbonates to a severe ball milling procedure. Unfortunately, this expensive and time consuming procedure does not adequately overcome the problem of homogenity even though the resulting size of the carbonate particles is satisfactory.
With the present invention, however, it has been found that the problem of homogenity with respect to the mixtures compositional content and particle size uniformity can be overcome through the use of the coprecipitation procedure of this invention in which mixtures of suitable alkaline earth carbonates are dissolved in a solvent. The solvent is then driven off to leave a homogenous mixture of uniformity sized carbonate materials which are especially useful as an emission coating for a thermionic tube cathodic element.